BOSTON – Lawyers for Boston Marathon bombing suspect Dzhokhar Tsarnaev asked a judge Wednesday to bar federal prosecutors from arguing that targeting the crowded athletic event is a factor a jury should consider when weighing his possible punishment.

Tsarnaev's lawyers said the decision to bomb the marathon shouldn't be an "aggravating factor" in determining whether he should receive the death penalty, because prosecutors also argue that Tsarnaev committed the offense after substantial planning to cause death and commit an act of terrorism.

"Stated differently, the allegation that Tsarnaev targeted the marathon is simply a more specific statement of the substantial planning allegation," Tsarnaev's lawyers wrote in a court filing.

Federal law requires the jury to reach its sentencing decision by weighing each aggravating factor cited by prosecutors against mitigating factors cited by the defense. Tsarnaev's lawyers said having duplicative aggravating factors "can have no other effect than to introduce arbitrariness and unfairness into the jury's sentencing deliberations."

Last week, the defense asked a judge to eliminate another aggravating factor cited by prosecutors: Tsarnaev's alleged betrayal of the United States. Tsarnaev's lawyers said prosecutors — by citing his status as a newly naturalized U.S. citizen — are implying that he is "more deserving of the death penalty" than a native-born person who commits the same crime.

Tsarnaev, 20, is awaiting trial in the 2013 bombing. Prosecutors allege that he and his 26-year-old brother, Tamerlan, planted two pressure cooker bombs near the finish line, killing three people and injuring more than 260. Tamerlan Tsarnaev died following a shootout with police several days after the bombing.

In a flurry of pretrial motions Wednesday, Tsarnaev's lawyers also asked the judge to declare the federal death penalty unconstitutional.

Tsarnaev's lawyers argue that the constitution's protection against cruel and unusual punishment prohibits the application of the death penalty because it is not authorized under state law in Massachusetts. They also argue that there is mounting evidence that innocent people have been executed in recent years and cite "worldwide revulsion over the recurring spectacle of botched executions," including recent ones in Ohio and Oklahoma.

But they also acknowledge that a federal appeals court rejected a challenge to the federal death penalty in another Massachusetts case in 2007.

Tsarnaev's lawyers also asked a judge to eliminate the death penalty as a possible punishment if they are able to show that prosecutors gave erroneous instructions to the grand jury that indicted him.

The defense wants to see the legal instructions given to the grand jury. Specifically, the defense notes that the 30-count indictment includes 17 counts that carry the possibility of the death penalty and also includes 26 special findings made up of factual allegations that must be established before a sentencing jury can consider imposing the death penalty.

Tsarnaev's lawyers say the grand jury should have been informed of the significance of the special findings, "namely that by returning such findings, the grand jury was authorizing the government to seek the death penalty," and for a jury to impose it.